It is known that container lids of thermoplastic provided with tear-up opening arrangement can be manufactured by injection moulding. The known container lids usually have a tearing indication or material weakening, the tearing up of which means that the portion of the lid located within the weakened area can be removed by folding up so as to expose an emptying opening when the contents within a container are to be made accessible. The known container lids usually are also provided with some type of pull-tab or pull-ring by means of which the tearing up of the tearing indication can be facilitated.
The requirement made on such container lids is, of course, that they should be inexpensive and easy to manufacture, but above all the opening arrangement should function well. This implies among other things that it should be easy to tear up the tearing indication without having to apply excessive foce, but that it should not be liable to open spontaneously during normal transport and handling of the container. These two requirements, as will be readily understood, are contradictory, since great security against spontaneous tearing up means that the tearing indication must not be made too weak, which in turn means that a greater force is required for tearing up the indication when the container is to be opened. In certain cases, depending on the contents of the container, it is also desirable, or even necessary, that it should be possible to reclose the opened container between each emptying operation and, from a point of view of convenience, the reclosure should be possible, preferably, by folding back the folded-up portion of the lid so that it will snap closed in its original position through engagement with the residual tearing edge in the opening contour. For such reclosing by snap engagement to function well the tearing edges of the torn-up tear indication ought to be straight and preferably completely unfrayed.
With conventional thermoplastics, such as e.g. polyethylene, which have been used hitherto as a starting material for injection moulding of the known container lids, it has proved difficult to obtain a well-functioning opening arrangement which is readily openable and which makes possible the desired snap closing on reclosure of the container. This difficulty is due to a large extent to the fact that thermoplastics are a very tough (elastic) material which offers great resistance to tearing on the tear indications, even in the case of small material thicknesses, and in order to make it possible for the tear indication to tear without excessive force having to be applied, the conventional thermoplastics consequently require that the injection moulding must be carried out under very strictly controlled conditions, so that an optimum material thickness within frequently very narrow tolerances is achieved with regard to the tearing indication. Such strict conditions, required during the injection moulding, are difficult to reconcile, however with commercial production on a large scale, and for economic reasons, therefore, it has been more or less inevitable to waive the requirement regarding optimum material thickness of the tear indication.